This invention relates to a method for making tape for tape automated bonding (TAB) for integrated circuits (ICs) and the tape product derived therefrom. More particularly, this invention relates to a novel method for producing three layer TAB materials comprising conductive foil, adhesive and polymer (polyimide) film wherein the feature window includes a removable support ring.
The general process of tape automated bonding for integrated circuits is known in the art. The TAB process requires production and use of a substrate having a feature window into which the integrated circuit is placed for connection to circuitry.
There are currently three types of tape automated bonding (TAB) materials including "one layer" (all metal), "two layer" (polyimide-metal), and "three layer" (polyimide: adhesive: metal).
Interest in TAB technology is on the rise among electronic manufacturers as an integrated circuit (IC) interconnection method because it offers several advantages over other technologies. One of these advantages is its ability to bond to very small closely spaced pads on the device using thermo-compression bonding techniques. When the TAB leads are long and narrow, however, it becomes increasingly difficult to ensure that they remain exactly as designed: planar and precisely located, so that they will align exactly with bond sites.
Because both two and three layer materials have nonconductive backing, both offer the potential for a method of support for long delicate TAB leads. In the case of two-layer material, where patterns are etched into both copper and polyimide layers, a support structure of virtually any shape, including free floating, is possible.
Three-layer material has had a more limited range of support structure available. Because features in the polyimide layer are formed in a mechanical punching operation prior to the addition of the copper layer, it had previously been impossible to offer a free floating annular support ring. Previously, rings had to be supported, typically by using corner tie bars.
Following material processing, immediately prior to outer lead bonding, chip and leads must be excised from the continuous tape. With prior art technology, tie bars have to be excised at the same time as outer leads. This requires tooling to pierce polyimide as well as copper, and a tool configuration more complex than the simple square design used with floating rings. In addition, tie bars require areas of the TAB design to be free of conductors, thus limiting the design freedom of the packaging engineer.
The problems and drawbacks of the support rings necessitating corner ties has been solved by a free floating annulus support ring disclosed in U.S. Pat. No. 4,571,354, assigned to the assignee hereof and incorporated herein by reference. The method of U.S. Pat. No. 4,571,354 uses a temporary carrier or support substrate to form the free-floating annular ring. The ring is held permanently in place when the copper layer is laminated to the polyimide, at which time the temporary carrier is removed. The support ring not only provides physical support for the delicate TAB interconnection leads, but it also aligns and registers them by fixing their position and maintaining the separation spaces between them.
Three-layer TAB materials offers significant advantages over two-layer materials. Heavier polyimide film, typically 5 mil thickness (1 mil=0.001 inch) aids in dimensional stability. The adhesive bond achieved between copper and polyimide layers is generally better than an adhesiveless bond. No secondary etch of the polyimide layer is required. The thermosetting phenolic butyral adhesive layer offers a high quality second dielectric layer, one which absorbs far less water (less than 0.1 percent) than the polyimide (three percent), and hence is a good barrier to leakage currents.
However, one important drawback of the three-layer TAB material of U.S. Pat. No. 4,571,354 (and indeed those three-layer TAB materials utilizing support rings of any type including corner ties) is that the support ring (e.g., polyimide) and the adhesive (acrylic, epoxy or phenolic butyral) are made of organic material. Furthermore, the support ring and adhesive can retain small quantities of water, and under some circumstances, there can be caused an outgassing of material from the organic materials.
In certain demanding applications, particularly in the military, it is desirable to avoid having an organic material contained within the IC package. Unfortunately, a dilemma exists in that these demanding applications also require the support ring for accurate inner lead and outer lead bonding.